PROJECT SUMMARY: Understanding the relationship between risk factors and early symptoms is crucial to early and differential diagnosis of Alzheimer?s disease (AD). Expression of the ?-4 allele of human apolipoprotein E (APOE-4) gene, a well-documented strongest genetic risk factor for development of the late-onset AD, associates tightly with the earliest AD symptom - olfactory deficit (OD) in humans. Animals expressing the human APOE-4 gene evince OD symptoms before AD pathogenesis, indicating a role of APOE-4 in functional disorders of the olfactory system. However, the pathophysiological mechanisms underlying the APOE-4 effects on olfaction remain unclear. We hypothesize that APOE-4 dysregulates neural circuits leading to excitation-inhibition imbalance and neural hyperactivity in the olfactory bulb (OB) to cause OD at the early stage of AD based on the following evidence. First, neuropathological changes of AD arise initially in the OB and its downstream brain centers. Second, humanized APOE-4 knock-in mice exhibit a behavioral OD coincidently with neuronal hyperactivity in the OB but not in the primary olfactory cortex at the age before AD pathogenesis. Third, the humanized APOE-4 animal genotype exhibits disrupted glutamatergic transmission via both presynaptic and postsynaptic mechanisms in multiple brain areas. The intrabulbar associational system (IAS) is formed by a subpopulation of glutamatergic neurons and play an essential role in regulating excitation-inhibition balance in the OB. Thus, APOE-4 potentially targets the IAS-intermediated glutamatergic synaptic transmission to disrupt the excitation/inhibition balance leading to neuronal hyperactivity in the OB. By capitalizing on the available progress and resource of the our currently active project, we propose to test the central hypothesis by focusing on the IAS with two specific aims. Aim 1: Investigate APOE-4 effects on the IAS-intermediated excitatory pathways within the OB. Optogenetic approaches will be employed to selectively label and activate IAS neurons for testing APOE-4 effects on excitatory transmission from OSNs to IAS neurons and IAS neurons to their postsynaptic targets. Aim 2: Determine APOE-4 impact on the IAS-driven inhibitory circuits in the OB. The inhibitory synaptic feedback to IAS neurons and OB output neurons as well as intrinsic properties of inhibitory OB interneurons will be compared between APOE-4 mice and control mice. This proposed work is within the scope of the currently supported project which focuses on the functional mechanisms underlying the IAS operation. Achievement of these two aims will advance our understanding how the OB is engaged in the early stage of AD at the cellular and circuit levels.